Although off-line analytical equipment exists for measuring physical dimensions of ophthalmic devices for quality control purposes, such “after-the-fact” results may be adequate to pass or fail certain lots of product, but they are not sufficient to make improvements during manufacture to reduce off-specification materials. Typically, an interferometer is relied upon for off-line analysis to detect surface irregularities. Such an interferometer relies on its own internal alignment system to take measurements.
In general, an interferometer works by splitting a beam of light into two separate beams: one reference beam is sent directly to a “reader sensor”, and the other test beam is directed to a lens or lens mold of interest, then into the “reader sensor.” The reference beam and test beam are then recombined. The difference between the reference beam and test beam is the interference created by the lens or lens mold of interest, thus providing an analog signal describing the effect of the lens or lens mold on a beam of light. Currently, an interferometer may be used as a tool to find alignment of the interferometer to a lens or lens mold of interest by using a portion of the light signal from the interferometer to produce a projected image onto a screen or detector. The interference between the reference beam and test beam are displayed and imaged on a detector. The projected image is distorted by the lens or lens mold of interest by bending the light to one side when it is not located over the center of the lens or lens mold. The light bends more as the distance from the center is increased, and the light bends in relation to the direction of motion needed. The projected image is then viewed with a vision system and compared to a round, sharp edged, circle. The position of the interferometer is moved in X, Y and Z until the projected image matches such a round, sharp circle. By moving the interferometer beam, the projected image will form a perfect circle when the light from the emitter is passing through the dead center of the lens or lens mold of interest. The projected image may also appear scattered or fuzzy due to variation in ‘focus’ by the lens or lens mold when it is not in its ideal height above the lens or lens mold. By locating the interferometer at the right height above the lens or lens mold, the beam of light passes through the lens or lens mold center with the least possible scattering of light; the edge of the projected image will become sharp and the size is small. The ideal image would be a perfect circle with no points or distortions outside the edge of the circles. There is one height where the projected image is the sharpest, and this is where the system must reside for ideal measurement with the interferometer.
Currently employed interferometer methods and systems, however, have many drawbacks. First, the interferometer has to be very well aligned with the lens mold or lens of interest. Current methods rely on the shape of the projected image to adjust the height and center point of the interferometer to align with the lens or lens mold of interest. To achieve proper positioning of the interferometer, current interferometer systems and methods require a second beam splitter for that beam, which induces some error by passing the beam through a lens or lens mold wherein the positions must be adjusted to account for the error introduced by the second beam splitter on the beam. In addition, adding the second beam splitter decreases the signal to the interferometer reader sensor. Therefore, a greater percentage of the signal sent to the positional ‘screen’ provides a better image having more contrast to adjust for location, but also removes contrast from the final interferometer image. Current systems and methods also require that the position of the projection screen must be perpendicular to the split beam to prevent the image from becoming oblong. Similarly, the position of the camera must be perpendicular to the screen to prevent the image from becoming distorted wherein the image will not appear to have a round shape. Failure to ensure that the position of the camera is perpendicular to the screen may result in an unadjustable error. Moreover, the occurrence of tolerance stack-up is unavoidable in the current multi-part systems and methods because errors accumulate through the use of multiple parts including a second beam splitter, projection screen and camera whereby the distortion created by each misaligned part is cumulative and results in a distorted final image. Thus, in the current systems and methods, tolerance stack-up must be corrected at each step of the positioning process. In addition, current methods provide a very limited alignment capability due to the rapid loss of the interferometer signal from the detector or screen.
Moreover, with a conventional phase-shifting laser interferometer, multiple frames of data are acquired over many milliseconds allowing enough time for vibration and turbulence caused by environmental factors to degrade the measurement results and its use under off-line conditions is adequate to provide pass/fail results.
Currently, the contact lens diameter may be measured after manufacturing including hydration, packaging and sterilization. This does not provide real time feedback to the manufacturing personnel to correct out of control conditions.
There is thus a need in the ophthalmic industry to improve manufacturing product quality by providing analytical equipment and techniques that can be implemented on the manufacturing line for use during a continuous or semi-continuous manufacturing process. Further, in order to measure a contact lens surface and center thickness, two measurement systems are needed, which adds complexity to the measuring. In multiple measurement systems there is a need to locate between a lens surface and the center thickness measurement device.